Circuiting Branching Laterals
To use the 12mm tube friction/barb loss calculation tables for branching or split run laterals use the critical circuit length method shown below Critical circuit length example
The critical length of this circuit for the calculation is the
length of line A and the circuit's total flow and the length
of the longest water path which is line C and its individual
branch flow. Calculation for line A = 50' and total flow for
the circuit which is 6 g.p.h. x 17 emitters or 102 g.p.h.
In the .580 tubing table we would use the minimum 100' length
chart and 12.5 foot spacing column. Our total flow was 102
g.p.h. We would use the 105 g.p.h line and calculate a loss of
.936 p.s.i. for line A.
For line C we have 8 emitters at 6 g.p.h. for a total of 48
g.p.h. for flow. Line C is 100' long so
we use the 100 foot chart again, using the 55g.p.h. line to be
on the conservative side. In the 12.5 foot spacing column, we
read .316 p.s.i. loss at that flow.
The completed circuit calculation would be:
Line A .936 P.S.I. Loss
Line C +.316 P.S.I. Loss
1.252 P.S.I. Loss for the Entire Lateral
The flow and pressure for line B were accounted for in the
Line A calculation when the total flow of the circuit was
used. Then when we figured up the pressure needed at the tee
to supply the Digger branch, we automatically took care of the
Friction Loss Calculation For Branching PVC Laterals
There are at least two or three methods for determining
friction loss in branching PVC pipe laterals. The first, and
one of the most accurate, is the critical circuit length
method. Again, the critical circuit length is the longest path
that the water must travel in the circuit. With this method
for PVC pipe, the outlet factors are not used.
Step number one in this method is to determine the pipe
sections in the circuit that make up the critical path.
Step number two is to calculate the friction loss for the flow
of each of these sections beginning with the last section and
working back toward the head of the circuit.
Step three is to add up these losses for a total P.S.I.
friction loss number for the circuit. In circuits of 400, or
less, friction loss through the various fittings is
negligible. However, if you wish to throw in a very high,
conservative loss factor, add another 25% to your total pipe
friction loss number.
Step four is to add in any additional loss of pressure due to
elevation increases or subtract any elevation gains from your
total P.S.I. loss due to friction for your final number.
The second method for branching PVC laterals is the short cut
method where a certain degree of accuracy is sacrificed in
exchange for a fastball park estimate of friction loss.
With this method you assume that all the pipe in the lateral
is in straight line. Total up all the
pipe section in your lateral, then calculate the friction loss
as if the total flow of the circuit went the entire length.
Count up the number of outlets and select the closest
multiplier to your actual outlet number from Table 1. Next
multiply your P.S.I. loss by the outlet factor for your
estimate of pipe friction loss. Add 25% onto your pipe loss to
adjust for fitting loss and you then have your final P.S.I.
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